Influenza virus type A is a member of the orthomyxoviridae family of viruses that cause influenza infection. The infective potential of influenza is frequently underestimated and can result in high morbidity and mortality rates, especially in elderly persons and in high-risk patients, such as the very young and the immuno-compromised. Influenza A viruses primarily infect the nasopharyngeal and oropharyngeal cavities and produce highly contagious, acute respiratory disease that results in significant morbidity and economic costs. Typical influenza viral infections in humans have a relatively short incubation period of one to two days, with symptoms that last about a week including an abrupt onset of fever, sore throat, cough, headache, myalgia, and malaise. When a subject is infected with a highly virulent strain of influenza these symptoms can progress rapidly to pneumonia and in some circumstances death. Pandemic outbreaks of highly virulent influenza present a serious risk to human and animal health worldwide.
The immunodominant antigens present on the surface of influenza viruses are hemagglutinin (HA or H) and neuraminidase (NA or N). Genetic reassortment between human and avian or swine influenza viruses can result in a novel virus with a hemagglutinin and/or neuraminidase against which humans lack immunity. In the 20th century, the pandemics of 1918, 1957, and 1968 were the result of such antigenic shifts. The avian and swine influenza outbreaks of the early 21st century caused by 2009 pandemic influenza A(H1N1) (H1N1pdm09) subtype influenza viruses, and their infection of humans, have created a new awareness of the pandemic potential of influenza viruses that circulate in domestic poultry and swine. The impact of a major influenza pandemic has been estimated to be up as many as 200,000 deaths, 730,000 hospitalizations, 42 million outpatient visits, and 50 million additional illnesses in the U.S. alone.
Thus, the need remains for tests that provide sensitive, specific detection of influenza types and subtypes in a relatively short time in order to permit rapid and effective treatment of an infected person. In addition, detection and characterization of novel viruses infecting humans and wild or domesticated animals are critical for detection and vaccination for emerging influenza viruses, including those with pandemic potential.